Cooperative relay based on exploiting hybrid ARQ

In order to compensate the primary performance degradation, which is caused by the co-existing secondary communications or the heavy fading of primary link, a cooperative relay scheme is proposed in cognitive radio systems by taking advantage of the opportunities that are arisen by the primary ARQ (Automatic Repeat-reQuest) retransmission. With the help of this scheme, nontrivial relay assistances for the primary retransmission are achieved without initialization phase. The cooperation among secondary users provides additional spatial diversity to facilitate the primary transmission. This can be accomplished with an oblivious primary system and without assuming any non-causal information about the primary data. The scheme are further extended to the scenarios where multiple cognitive pairs exist. Performance analyses of the schemes are devised and the effectiveness is verified via simulations.